1. Field of the Invention
The invention relates to a production method of an n-type SiC single crystal, an n-type SiC single crystal obtained by that method, and application thereof. More particularly, the invention relates to a production method of an n-type SiC single crystal in which gallium (Ga) and nitrogen (N), which is to be a donor element for obtaining an n-type semiconductor, are added during crystal growth, an n-type SiC single crystal obtained by that method, and application thereof.
2. Description of the Related Art
SiC single crystal is extremely thermally and chemically stable, has superior mechanical strength and is resistant to radiation. In addition, SiC single crystal has superior properties as compared with silicon (Si) single crystal, including high dielectric breakdown voltage and high thermal conductivity. Moreover, SiC single crystal is able to easily control the type of electron conduction to either p-type or n-type conduction depending on the impurity added, and also has the characteristic of a wide band gap (about 3.3 eV for 4H—SiC single crystal and about 3.0 eV for 6H—SiC single crystal). Consequently, SiC single crystal is able to realize high temperatures, high frequencies, withstand voltage and environmental resistance unable to be realized with conventional semiconductor materials such as Si single crystal or gallium arsenide (GaAs) single crystal, thus making it the target of growing expectations for use as a next-generation semiconductor material.
Methods have been proposed for obtaining p-type and n-type conduction SiC, single crystal for semiconductor materials that includes introducing an impurity during crystal growth, and studies are being conducted on SiC single crystal and production methods thereof that contain various impurities. Lowering resistance when current is applied that causes power loss is an important factor in semiconductor devices such as switching devices, which are considered to be one of the main applications of SiC single crystal.
Japanese Patent Application Publication No. 6-219898 (JP-A-6-219898) describes a method for producing n-type 6H—SiC single crystal by a sublimation method in which 20 to 100 ppm of Al are added to SiC powder followed by sublimation in a nitrogen gas atmosphere. This publication also describes a specific example of an n-type 6H—SiC single crystal obtained according to the method described above that has resistivity of 0.1 Ωcm. In addition, Japanese Patent Application Publication No. 2002-57109 (JP-A-2002-57109) describes a method for producing SiC for use as a p-type semiconductor or n-type semiconductor that has a step for forming an Si layer on a substrate, a step for adding to the Si layer an impurity which is at least one of element selected from the group consisting of N, B, Al, Ga, In, P, As, Sb, Se, Zn, O, Au, V, Er, Ge and Fe, and a step for forming an SiC layer to which an impurity has been added by carbonizing the Si layer to which the impurity has been added. In addition, JP-A-2002-57109 also describes SiC in which the impurity concentration is within the range of 1×1013/cm3 to 1×1021/cm3, and that resistivity may be increased by simultaneously adding a donor and an acceptor.
In addition, Japanese Patent Application Publication No. 2003-73194 (JP-A-2003-73194) describes a Method for producing SiC single crystal preferable for use as a p-type semiconductor. The production method includes sublimating an SIC powder, in which the nitrogen (N) content is 0.1 ppm or less and the total content of an element belonging to group 13 of the periodic table, such as B, Al, Ga, In or Tl, is equal to or greater than the nitrogen content, followed by growing SiC single crystal by recrystallization. In addition, JP-A-2003-73194 describes SiC single crystal in which volume resistivity is 1×101 Ωcm or less. This publication also describes a specific example of a p-type SiC single crystal obtained according to this method in which the content of Al, which is an element of group 13 of the periodic table, is 40 ppm and the N content is 0.05 ppm or less.
In addition, Japanese Patent Application Publication No. 2005-109408 (JP-A-2005-109408) describes an SiC epitaxial growth method and an SiC epitaxially grown film for fabricating a device having low on resistance and high withstand voltage, by controlling so that the SiC epitaxially grown film is doped with at least one of N, B, Al and P within a range of 5×1023 cm−3 to 3×1019 cm−3. Moreover, Japanese Patent Application Publication No 2007-320790 (JP-A-2007-320790) describes a method for producing SiC single crystal by doping SIC single crystal with a donor and an acceptor, an SiC single crystal ingot fabricated according to this production method, and a substrate that uses the SIC single crystal ingot. The production method includes introducing a gas source, which includes an impurity containing an element such as N serving as the donor and an impurity containing one or both of elements such as B or Al serving as the acceptor, into a crystal growth atmosphere. This publication also describes a specific example of SiC single crystal obtained according to the above method that has a nitrogen (N) concentration of 7×1017 cm−3 or 9×1017 cm−3.
However, it is necessary to add a large amount of nitrogen (N) in order to obtain n-type SiC single crystal that has low specific resistance by these conventional SiC single crystal production methods. As a result, the concentration of nitrogen (N) in the n-type SiC single crystal becomes excessively high and ends up impairing the inherent characteristics of SiC single crystal. In SIC single crystal that contains nitrogen (N), which is a donor element, because a portion of the Si or C that composes the SIC single crystal undergoes a change in atomic radius as a result of being replaced by nitrogen (N), dislocation (crystal defects, crystal distortion) occurs in the portion of the SiC single crystal that has been replaced by the above elements. The dislocation has an effect on the inherent properties of the SiC single crystal (for example, by causing an increase in leakage current), thereby impairing the inherent characteristics of the SiC single crystal. Consequently, n-type SiC single crystal is produced by establishing an upper limit on the doped amount of nitrogen (N), and n-type SiC single crystal provided for industrial use has specific resistance of 0.015 Ωcm to 0.028 Ωcm.